This invention concerns a sorting machine for effecting relative separation between objects which have and those which do not have a predetermined characteristic, and thus between desired and undesired objects.
The term "objects" is used in this specification in a wide sense as including agricultural products such as rice, peas and beans, and minerals both in the form of fine particles and in the form of lumps of ore. The term "characteristic" is also used in this specification in a wide sense as including colour, reflectivity, light transmission, conductivity, magnetism, fluorescence, and emissivity.
In previously known sorting machines in which detectors are provided for examining a plurality of objects arranged side by side on a support, each detector has been arranged to examine a group of such objects. Consequently the response of the detector to the characteristic of the objects being examined has been determined by the average value of the said characteristic in the said group of objects. Such an average value, however, does not necessarily provide a good criterion for sorting.
For example, if the purpose of the sorting is to remove dark objects, the detector may be "fooled" when it simultaneously views a group of objects which include both a dark object, which should be removed, and several light objects, since the average colour of this group may be acceptable despite the presence of the dark object. A similar difficulty may occur if each support carries only a single file of objects, since in that case an object having a dark spot in one portion and a light spot in another portion will not necessarily be removed.
Sorting machines, moreover, commonly view objects to be sorted against a background whose colour is matched to the average colour of the objects being sorted. Such a background provides a compensation for the effects arising from variation in the size of the objects being sorted. However, the effective colour of both the background and the objects is liable to change. Thus, for example, the background and the objects may be illuminated by respective fluorescent tubes which age at different rates and/or which have aged unevenly along their lengths in different ways. The ratio between the light outputs of such fluorescent tubes, moreover, may change due to different amounts of dust building up thereon. Furthermore, although the average colour of the objects being sorted normally remains substantially constant, nevertheless even where the product being sorted remains the same, occasional changes in the average colour do occur which are normally slow. If, however, the product being sorted is changed to a similar but different product, such changes in the average colour are more probable.
If, moreover, the sorting machine is a multi-channel machine, the objects passing through the various channels are not all necessarily of the same average colour, nor are the backgrounds of the various channels necessarily of the same effective colour for the reasons discussed above.